MODULATED DIFFERENTIAL SCANNING CALORIMETRY IN THE GLASS-TRANSITION REGION .5. ACTIVATION-ENERGIES AND RELAXATION-TIMES OF POLY(ETHYLENE TEREPHTHALATE)S

Temperature-modulated differential scanning calorimetry is used to eva luate the kinetics of the glass transition from measurement of the fir st harmonic of the apparent, reversing heat capacity. The data are tak en from quasi-isothermal experiments with negligible instrument lag, e xtrapolated to zero modulation amplitude. Equations based on irreversi ble thermodynamics that can be understood in terms of the hole theory of liquids are applied to measurements on amorphous, semicrystalline, and biaxially drawn poly(ethylene terephthalate)s (PET). The activatio n energy of amorphous PET decreases from 328 to 153 kJ/mol on crystall ization and to 111 kJ/mol on orientation, and is correlated with an in crease in the preexponential factor. After annealing of the crystalliz ed samples below the glass transition temperature, the activation ener gy of the semicrystalline PET can recover beyond the level of amorphou s PET, to 387 kJ/mol. The earlier observed decrease in enthalpy relaxa tion on crystallization is linked to this sharp decrease in activation energy. (C) 1996 John Wiley & Sons, Inc.